1. Field of the Invention
This invention relates generally to a transmission for a motor vehicle, and, more particularly, to adaptively controlling the magnitude of torque transmitted by an input clutch.
2. Description of the Prior Art
A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
The magnitude of torque being transmitted through each input clutch of a powershift transmission, specially such a transmission having dry input clutches, can vary in a wide range when launching the vehicle from rest and while making gear ratio changes because the coefficient of dry friction of the clutch changes with variations in clutch temperature. Therefore, extensive adaptive control of the input clutch torque is required to achieve consistent operating feel to the driver during vehicle launches and gear shifts.
Engine torque is often used as feedback variable to control adaptively input clutch torque to the changing operating environment of the input clutches and vehicle. Experience has shown, however, that the variation of clutch torque can be equal to or greater than the error in estimating the magnitude of torque being produced by the engine, thereby adversely affecting the quality of the control of input clutch torque when engine torque is the control variable. Due to the engine torque estimation error currently available, input clutch torque varies in an unacceptably wide range and fails to produce a consistent vehicle launch and shift feel.
A need exists in the industry for a reliable, consistent technique for controlling input clutch torque in a powershift transmission that accounts for changing vehicle conditions and operating parameters.